This invention concerns a method of forming a colored coating film on cross-linked polyethylene and, more specifically, a method of forming a colored coating film having a firm adherence to the surface of cross-linked polyethylene under the irradiation of ultra-violet rays (hereinafter simply referred to as UV-rays). It also relates to a cross-linked polyethylene insulated wire.
Polyethylene can be cross-linked using an organic peroxide or adding water to silanes or under the irradiation of electron rays and they have been widely utilized, particularly, as insulating materials for electric wires, cables and the like due to their excellent electrical, mechanical and thermal properties.
So-called polyolefinic materials such as polyethylene and polypropylene including cross-linked polyethylenes are generally non-polar substance having inactive surfaces. Accordingly, it has generally been considered so far that formation of a colored coating film with a firm adherence to such a polyoelefinic material is extremely difficult. Particularly, since cross-linked polyethylenes are less soluble even in usual organic solvents and carbonize without melting above the thermal decomposition temperature upon heating, it has been considered almost inevitable to obtain a colored coating surface with a good adherence by using conventional solvent type paints.
Accordingly, in the case of a color distinction of electric wire insulated with a cross-linked polyethylene there have been adopted, for example, a method of compounding a coloring pigment together with a polyethylene resin upon extrusion of polyethylene insulator before cross-linking or a method of inserting a colored tape such as of a nylon tape between an electrical conductor and a polyethylene insulator for later cross-linkining, etc. However, a number of extra working steps are required for changing color and steps are complicated in the former method, while a troublesome work of inserting the tapes and provision for the stocks of tapes on various colors are required in the latter method. Accordingly, the above-mentioned conventional methods are disadvantageous in an economical point of view and reducing the production efficiency. In view of the above, a more economical surface color coating method of a solvent type paint using a vehicle on the surface of cross-linked polyethylene has been proposed and put to practical use as the most effective way, since color-distinguished electric cables of required length can effectively be produced without the complexity of color change works upon extrusion.
That is, in this surface coloring method, a paint which is prepared by dissolving a resin such as cyclized rubber, polyamide and vinyl chloride as a vehicle together with a coloring pigment into a solvent is coated and then spontaneously dried or heated above the melting point of the vehicle simultaneous to evaporate the solvent thereby applying the coloring pigment together with the vehicle on the surface of the cross-linked polyethylene. However, the adherence of the coating paint with the cross-linked polyethylene is still poor in such a surface coloring method and, for instance, peeling is resulted to the coating film by the felt-friction test under the load of 500 g after reciprocating friction from about 10 to several tens cycles and the colored paint is liable to be peeled off in a relatively short period of time when it is used in such an application as electric wires, cables and the like that undergo frequent bending or friction. Furthermore, coloring may some time be indistinct and, in addition, circumstantial pollutions may possibly arise due to the evaporation of the solvent in the paint during production. Therefore, even this method is not quite satisfactory.
In view of the above, a method of using a paint prepared by blending a coloring pigment with a radiation ray curable resin has recently been proposed as a coloring method for polyolefinic material for the solvent type paint.
For instance, Japanese Patent Publication No. 7966/1975 discloses a coloring method which comprises coating an ink consisting of a polyfunctional monomer having two or more reactive functional groups and a pigment to the surface of plastic material as an object to be colored, polymerizing the monomer under the irradiation of radiation rays (electron rays) and chemically bonding the monomer with the plastic.
In the case of polyethylene, however, since it is necessary to previously add the monomer at least to the surface thereof prior to the coating of the ink in this method, the pre-treating step therefor is complicated. In addition, a large installation cost is required for the irradiation device used for coating and, further, the processing step for the irradiation of radiation rays has to be carried out in the absence of oxygen, for example, under an inert gas atmosphere.